1. Field of the Invention
This invention relates to an analog/digital converter circuit for converting an analog signal into a digital signal and, more particularly, to an analog/digital converter circuit for use in a video signal processing circuit or the like.
2. Description of the Related Art
Recently, in the television receivers, various control operations can be omitted and special functions can be added by properly processing digital signals.
FIG. 1 is a circuit diagram of conventional analog/digital converter circuit (which is hereinafter referred to as an A/D converter circuit) which is formed as an integrated unit and used in a digital television receiver to convert the Y signal (luminance signal) or the R-Y or G-Y signal (color difference signal) into a digital signal. A circuit portion surrounded by dot-dash lines in FIG. 1 is an internal circuit portion of integrated circuit 40 in which are formed input buffer circuit 41 for receiving an analog voltage, A/D converter section 42 for converting the output of input buffer 41 into a digital signal, and analog switch 43.
Reference voltage generating circuit 46, voltage comparator circuit 48, and decoder circuit 49 are formed in A/D converter section 42. Reference voltage generating circuit 46 includes a plurality of resistors 44 and constant current source 45 connected between power source voltage terminal V.sub.DD and ground voltage terminal V.sub.SS, and voltage comparator circuit 48 includes a plurality of voltage comparators 47.
Analog signal Vin, such as the Y signal or the R-Y or G-Y signal, is supplied to input buffer circuit 41 of integrated circuit 40 via coupling capacitor 50 provided outside circuit 40. The output of input buffer circuit 41 is supplied to one input terminal of each of voltage comparators 47 of voltage comparator circuit 48, the other input terminal of each of voltage comparators 47 is supplied with a corresponding one of different reference signals which are generated at respective nodes between resistors 44 in reference voltage generating circuit 46. Decoder circuit 49 outputs digital data corresponding to comparison outputs of voltage comparators 47.
In a case where the Y signal or the R-Y or G-Y signal is converted into a corresponding digital signal, it is necessary to set the range of variation in input analog voltage Vin to equal to the dynamic range of the A/D converter circuit, so as to utilize effectively the dynamic range of the A/D converter circuit. For this reason, the conventional A/D converter circuit is provided with a pair of resistors 51 and 52 for dividing a voltage between power source voltage terminals V.sub.CC and V.sub.EE which are located outside the integrated circuit. The voltage divided by the pair of resistors 51 and 52 is supplied to emitter follower circuit 55, constituted by npn transistor 53 and resistor 54, and the output voltage of emitter follower circuit 55 is supplied to one end of analog switch 43. In the A/D converting operation, analog switch 43 is turned on by control signal S for a preset period of time so as to permit the divided voltage to be supplied to the input terminal of input buffer circuit 41. The lowest voltage in the dynamic range of the A/D converter circuit is set based on the resistance division ratio of resistors 51 and 52, as a D.C. bias voltage in a case where analog voltage Vin is the Y signal, and the intermediate voltage of the dynamic range of the A/D converter circuit is set based on the resistance division ratio of resistors 51 and 52, as the D.C. bias voltage in a case where analog voltage Vin is the R-Y or G-Y signal.
If the D.C. bias voltage is derived by resistance division as in the conventional circuit, the bias voltage derived will deviate from the initial value by virtue of variation in the resistances of resistors 51 and 52, variation in the characteristic of transistor 53 due to temperature change, variation in power source voltage V.sub.CC or V.sub.EE or the like. For this reason, it is then necessary to set the variation range of the analog voltage with a margin with respect to the dynamic range of the A/D converter circuit. Thus, the conventional A/D converter circuit has a defect in that the dynamic range of the A/D converter circuit cannot be used most effectively.